Plants play a vital role in the cycle of life on Earth, creating both the oxygen and food that many species need to survive. Plant species create simple sugars, such as glucose and fructose, and starches that they use in different ways depending on their needs. To do this, they use the chlorophyll in their leaves or leaf equivalents to convert water, sunlight and carbon dioxide into a simple sugar, which the plant uses immediately or stores for later use. Plant life’s two distinct strategies for storing excess sugar act as food production for other creatures – like humans.

Plants contain a system for the movement of water and a system for the movement of energy called the xylem and the phloem, respectively. For photosynthesis to occur, a plant must move water to its leaves through the xylem, a series of small, branching tubes that move water from the roots to the leaves. After a plant uses the building blocks of photosynthesis to make its food, it uses its phloem to move the glucose created to its branches, roots, trunk and fruits.

Photosynthesis creates glucose, which acts as the base of other more complex sugars found in plants. For example, fructose, or fruit sugar, shares a structure similar to glucose, but it is used in different parts of the plant. As simple sugars are water soluble, plants can access and use them easily. Glucose appears in the stems of some plants, like the corn plant, while fructose, as its name implies, appears commonly in fruit. Humans and other animals often eat these foods to gain these basic units of chemical energy.

Starch acts as a form of reserve energy in plants. Plants contain two types of starch – amylose and amylopectin – both polysaccharides or combinations of sugar molecules. In some cases, it takes thousands of sugar molecules to form a starch. Roots, legumes and seeds commonly contain starches, the latter case because starch feeds the embryonic stage of a plant. Animals use their digestive enzymes to break down starches into simple sugars for use. Foods like potatoes contain high levels of the sugar chains. Other polysaccharides, like cellulose, give plants structure, providing walls for their cells.

Compared to sugars, lipids and fats have a relatively high nutritional density. However, plants tend to favor sugars as an energy source, although lipids can be found in the seeds of some species. Some scientists hope to increase the concentration of lipids in plants both as a food and fuel source. The reason plants don't use lipids as energy, some researchers believe, is because plants have evolved specifically to use sugars for so long.